1. Field of Invention
The present invention relates to optical substrates having a structured surface, particularly to optical substrates for brightness enhancement and diffusion, and more particularly to brightness enhancement and diffusion substrates for use in flat panel displays having a planar light source.
2. Description of Related Art
Flat panel display technology is commonly used in television displays, computer displays, and displays in handheld electronics (e.g., cellular phones, personal digital assistants (PDAs), digital cameras, tablets, etc.). Liquid crystal display (LCD) is a type of flat panel display, which deploys a liquid crystal (LC) module having an array of pixels to render an image.
FIG. 1 illustrates an example of an LCD display. A backlight LCD 10 comprises a liquid crystal (LC) display module 12, a planar light source in the form of a backlight module 14, and a number of optical films interposed between the LC module 12 and the backlight module 14. The LC module 12 comprises liquid crystals sandwiched between two transparent substrates, and control circuitry defining a two-dimensional array of pixels. The backlight module 14 provides planar light distribution, either of the backlit type in which the light source extends over a plane, or of the edge-lit type as shown in FIG. 1, in which a linear light source 16 is provided at an edge of a light guide 18. A reflector 20 is provided to direct light from the linear light source 16 through the edge of the light guide 18 into the light guide 18. The light guide 18 is structured (e.g., with a tapered plate and light reflective and/or scattering surfaces 30 defined on the bottom surface facing away from the LC module 12) to distribute and direct light through the top planar surface facing towards LC module 12. The optical films may include upper and lower diffuser films 22 and 24 that diffuse light from the planar surface of the light guide 18. The optical films further includes upper and lower structured surface, optical substrates 26 and 28, which redistribute the light passing through such that the distribution of the light exiting the films is directed more along the normal to the surface of the films. The optical substrates 26 and 28 are often referred in the art as luminance or brightness enhancement films, light redirecting films, and directional diffusing films. The light entering the LC module 12 through such a combination of optical films is uniform spatially over the planar area of the LC module 12 and has relatively strong normal light intensity.
The main function of brightness enhancement films 26 and 28 is to improve the brightness of overall backlight module. The effect of brightness enhancement films is to increase the amount of light emitted at small angles to the axis of the display by reducing the amount emitted at greater angles. Thus, as one looks at a display at increasing angles with respect to the axis, the perceived brightness will decline. Between 35 and 45 degrees the perceived brightness will decline very rapidly. This effect is known as a sharp cutoff.
In the backlight LCD 10, brightness enhancement films 26 and 28 use longitudinal prismatic structures to direct light along the viewing axes (i.e., normal to the display), which enhances the brightness of the light viewed by the user of the display and which allows the system to use less power to create a desired level of on-axis illumination. The brightness enhancement films 26 and 28 have a light input surface that is smooth or glossy, through which light enters from the backlight module. Heretofore, many LCDs used two brightness enhancement film layers (as in the LCD in FIG. 1) that are rotated about an axis perpendicular to the plane of the films, relative to each other such that the longitudinal peaks/grooves in the respective film layers are at 90 degrees relative to each other, thereby collimating light along two planes orthogonal to the light output surface.
When the glossy bottom surface of the brightness enhancement film 26 above the structured surface of the other brightness enhancement film 28, it has been experienced that the optical interaction between the glossy surface of top brightness enhancement film 26 and the structured surface and/or glossy surface of the lower brightness enhancement film 28 creates undesirable visible artifacts in the display image in the form of interference fringes (i.e., bright and dark repeated patterns) that are observable in the display image. These bright and dark patterns may also be generated between the upper brightness enhancement film 26 and the adjacent surface of the LC module 12 absenting an upper diffuser film 22 (FIG. 1). Undesirable image affecting effects arising from flaws and non-uniformities such as interference fringes, cutoff effects (rainbow), physical defects, flows, stains, can be masked by using an upper diffuser film (e.g., diffuser film 22 above brightness enhancement film 26 in FIG. 1).
There is an increasing need for reducing power consumption, thickness and weight of LCDs, without compromising display quality of the LCDs. Accordingly, there is a need to reduce power consumption, weight and thickness of backlight modules, as well as thicknesses of the various optical films. In this regard, many light directing techniques have been developed to reduce power consumption without compromising display brightness. Some developments are directed to the design of the backlight module (i.e., designing structures of the components of the backlight module 14 in FIG. 1, comprising the light source 16 and reflector 20, and light guide 18, to improve overall light output performance. In addition, other developments are directed to diffuser films 22 and 24, and luminance/brightness enhancement films 26 and 28.
Heretofore, to reduce the overall thickness of the optical films in LCDs, much effort had been directed to reducing the number of the optical films, from four films (e.g., optical films 22, 24, 26 and 28 in FIG. 1) to three films. In this regard, one approach is to keep the low diffuser film 24 and low brightness enhancement film 28 as separate structures, but the functions of the top diffuser film 22 and top brightness enhancement film 26 are combined and merged into a single hybrid film structure. The three-film type display has been widely adopted in handheld electronic devices and notebooks, where it is particularly desirable to push the envelope to reduce overall size of such devices.
Various efforts also have been undertaken to develop hybrid brightness enhancement films. Referring to FIG. 2, U.S. Pat. No. 5,995,288 disclosed a coating layer of particles provided on the underside of the optical substrate, on the opposite side of the substrate with respect to the structured surface on the top side. A glossy surface is no longer present at the underside of the optical substrate. The added particles achieve the effect of scattering light for light diffusion. Referring to FIG. 3, U.S. Pat. No. 5,598,280 disclosed a method to form small projections on the underside of the optical substrate to improve uniformity in luminance by light diffusion. Such diffusion treatments will hide many of the interference fringes, making them invisible to the user. One of the disadvantages of these approaches is that light scattering decreases on-axis gain. Moreover, the hybrid brightness enhancement films are also less effective in directing light within the desirable viewing angle.
Others have explored modifying the structure of prism surface of the structured surface of the optical substrate. For example, referring to FIGS. 4A and 4B, U.S. Pat. No. 6,798,574 provides fine protrusions on the prism surface of the structured surface of the optical substrate, which is supposed to spread light in a certain direction with a wider angle.
Accordingly, all the foregoing hybrid brightness enhancement films involve weakened light output directivity. Moreover, the overall brightness or luminance for the foregoing films is significantly reduced. Further, all the above-mentioned hybrid brightness enhancement films involve relatively complex structures requiring relatively higher manufacturing costs.
There remains a need for an optical substrate having a structure that both enhances brightness and provides effective diffusion, and overcoming the shortcomings of the prior art multifunctional optical films.